1. Field of the Invention
The present invention generally relates to an optical fiber transmission system, and in particular to an optical transmitter system coupled to optical fiber for use in the transmission system.
2. Description of the Related Art
Nonlinear effects in fiber may have a significant impact on the performance of optical fiber communication systems. One of the most significant effects may be caused by Stimulated Brillouin Scattering (SBS). When SBS occurs, the backscattering wave propagates in the opposite direction of the input light, resulting in attenuation of light at the receiving end which would not be compensated for by the input light increasing in power. Further, SBS may lead to distortion and noise, affecting the bit error rate at the receiver.
It has been known that SBS occurs when the level of input light exceeds a certain threshold P.sub.TH which is proportional to (.DELTA.V.sub.SBS +.DELTA.V.sub.LD)/.DELTA.V.sub.SBS, where .DELTA.V.sub.SBS is Brillouin bandwidth and .DELTA.V.sub.LD is the spectral line width of signal light input into an optical fiber. In the case of a system employing an external modulator coupled to semiconductor diode laser operating at a wavelength of 1.55 .mu.m, the threshold P.sub.TH is on the order of several decibels (dB), which probably leads to SBS in optical fiber amplifier systems.
To suppress SBS, the semiconductor laser is driven by a bias current on which a frequency-modulation signal is superimposed as an SBS suppression signal. Such a SBS suppression method has been proposed by Y. Aoki et al ("Stimulated Brillouin Scattering Suppression in IM/DD Optical Fiber Communication System with Optical Booster Amplifiers", IEICE, Nov. 21, 1991, pp75-80). By applying a frequency-modulation signal, chirping occurs in the output light of the semiconductor laser. Since chirping causes the spectral line width .DELTA.V.sub.LD to be broadened, the threshold level P.sub.TH becomes higher (see FIG. 3 of page 76 in the above paper by Y. Aoki et al.). That is, the larger the superimposed level of the frequency-modulation signal, or the SBS suppression signal, the higher the threshold level P.sub.TH at which SBS starts occurring. However, in the case of the frequency-modulation signal of great amplitude, the wave form of laser light is degraded, leading to a degradation of transmission performance. Therefore, it is important to adjust the modulation degree of SBS suppression signal. More specifically, the SBS suppression signal should be set to a minimum level at which SBS does not occur.
Several disadvantages exist in the case of a wavelength-division multiplexing (WDM) system where a plurality of wavelength signals are combined by an optical coupler and then amplified by an optical fiber amplifier. The output light of the optical fiber amplifier includes variations in signal level from wavelength to wavelength because of variations in-output light level from semiconductor laser to semiconductor laser, variations in loss from port to port of the optical coupler, and the gain characteristic of the optical fiber amplifier.
In conventional optical transmitters for use in WDM systems, the superimposed level of the SBS suppression signal is set for each semiconductor laser so as to be matched with the input light having the maximum power which is estimated from the above variations and characteristics. Therefore, the wave forms of laser signals other than the maximum power signal are degraded due to excessive superimposed level of the SBS suppression signal, leading to a degradation of transmission performance.